Yeah,

13V cuttoff sounds right. Most charging systems go to at least 13.5
volts and batteries seldom settle near 13 when not charging. 1 ma is
fine. FET? is that what it takes? Its gota work for low and high loads
same...

RST Engineering wrote:
What do you want the cutoff voltage to be? Personally, I'd pick 13.0 volts,
but it is child's play to do any voltage you want.

There will be about a milliampere of quiescent current for the sensor that
will still be on-line when the output shuts off.

Jim